Not Applicable
The present invention relates generally to optical communication systems and more particularly to electro-mechanical systems (MEMS) devices and methods of fabricating the same.
Optical communication systems are increasingly being used to communicate data, voice, multimedia and/or other communications. Optical communication systems may employ optical fibers and/or free space optical communication paths. It will be understood by those having skill in the art that optical communication systems may use optical radiation in the visible, ultraviolet, infrared and/or other portions of the electromagnetic radiation spectrum.
Reflectors, such as mirrors, are widely used in optical communications systems. For example, optical cross-connect switches often include an array of reflectors to reflect optical energy from any switch input to any switch output. Similarly, many add-drop optical switches, wavelength blockers, and/or dynamic gain equalizers also use an array of reflectors such as mirrors to couple various optical paths.
It has been proposed to fabricate arrays of reflectors using micro-electro-mechanical system (MEMS) technology. As is well known to those having skill in the art, MEMS devices are potentially low cost devices, due to the use of microelectronic fabrication techniques. New functionality also may be provided, because MEMS devices can be much smaller than conventional electro-mechanical devices.
Many of the fabrication processes for MEMS, called micromachining, are borrowed from the integrated circuit industry, where semiconductor devices are fabricated using a sequence of patterning, deposition, and etch steps (e.g., on silicon). Silicon micromachining has been utilized since the early 1960s. At its early stage, bulk silicon micromachining was employed in the majority of the research efforts by etching away the bulk of the silicon wafer. Bulk micromachining was first practiced using anisotropic wet chemical etches, such as potassium hydroxide, which preferentially etch faster in certain crystallographic planes of single-crystal silicon. In the early 1980s, surface micromachining using sacrificial etching gave rise to new types of microsensors and microactuators. Typically, surface micromachining has used a deposited layer of polysilicon as the structural micro-mechanical material, which is deposited over a sacrificial layer onto a substrate, which is typically silicon, such that when the sacrificial layer is removed, the polysilicon remains free standing. Recent advancements in reactive ion etching (RIE) technology have made practical, and in many ways preferential, the use of dry plasma etching to define micro-mechanical structures. Reactive ion etching techniques are independent of crystal orientation, and can create devices exceeding the functionality of surface micro-machined devices. The use of single-crystal materials, particularly silicon, can be beneficial for mechanical applications because of the lack of defects and grain boundaries, maintaining excellent structural properties even as the size of the device shrinks. Fabrication techniques involving the bonding of two separate single-crystal wafers also have been proposed, wherein one wafer serves as the substrate and another wafer forms the structural micro-mechanical material/layer.
For many reflective MEMs applications, it is desirable that the reflectors are electrostatically actuated. Electrostatic actuation provides effective analog positioning and tuning. Furthermore, electrostatic actuators are relatively easy to fabricate and provide high operational speeds due to their relatively small mass. This is in contrast to other actuators, such as piezoelectric actuators, which are typically much heavier. Electrostatic actuation of a structure is typically accomplished by applying a voltage between an electrode on the structure and an electrode separated from the structure. The resulting attractive electrostatic force between the electrodes enables actuation of the structure toward the separated electrode. This applied electrostatic force is opposed by a characteristic mechanical restoring force that is a function of the structure""s geometric and materials properties. However, the electrostatic force is a nonlinear function of distance. As the structure moves toward the separated electrode, such that the electrodes"" separation distance decreases, the electrostatic force between the electrodes typically increases superlinearly. In contrast, the mechanical restoring force of the structure typically is a linear function of distance. Accordingly, not all positions between the electrodes are stable. In particular, when the air-gap between electrodes reaches a minimum spacing characteristic of the structure, the structure position is unstable and causes uncontrollable travel of the structure through the remaining distance to the separated electrode. This instability condition is generally known as xe2x80x9cpull-inxe2x80x9d, and can result in stiction (i.e., where the reflector is stuck to the electrode) and/or actuator deformation. This pull-in phenomenon typically reduces the actuation range of electrostatic MEMS devices.
An example of a reflector array, which can be fabricated from the above processes, is shown in FIG. 1. The micro-mechanical structure 10 includes an array of single axis mirrors 12 disposed about a common rotation axis 14, which is parallel to the array axis. This type of structure is commonly referred to as a piano MEMS array. Each mirror 12a is suspended above a substrate 20 with a torsional hinge 16a, which ideally is coaxial with the rotation axis 14, such that the mirror 12a is able to pivot about the rotation axis 14. Each end of the torsional hinge 16a is connected to a mechanical anchor 18a. 
Referring to FIGS. 2a and 2b, the pivotal movement of each mirror 12a in FIG. 1 is electrostatically actuated by first 22 and second 24 lower electrodes deposited on the substrate 20, under the mirror 12a. In particular, a voltage applied between an upper electrode (i.e., the micro-mirror) and a first underlying electrode 22 will tilt the mirror in a first direction, while a voltage applied between the upper electrode and a second underlying electrode 24 will tilt the mirror in a second opposite direction, as shown by the dotted lines in FIG. 2b. 
Unfortunately, the single-axis design illustrated in FIGS. 1 and 2a,b is associated with a number of disadvantages. In general, these disadvantages are related to the fact that the micro-mirrors (i.e., the optical components) are part of the actuators (i.e., the electromechanical components), and thus the optimization of each component is compromised. For example, in terms of optimizing the optical design it is often desired to have a micro-mirror size that is large enough to facilitate alignment and reduce insertion loss. In practice, the electromechanical requirements can limit the size of the optical components, since it is faster and requires less energy to move a lighter object. Similarly, using the piano MEMS shown in FIGS. 2a and 2b as an example, a longer arm provides a stronger electromechanical design due to increased leverage and/or torque and thus less actuation energy, whereas a shorter arm provides a stronger optical design, since it provides greater rotation angles and/or reduces the effect of pull-in. Unfortunately, these contrasting requirements can introduce challenges in the MEMS design.
U.S. Pat. No. 6,480,320 to Nasiri discloses a micro-electromechanical mirror and mirror array that addressed some of these concerns. However, in this micro-electromechanical device, the mirror is supported on a post above the actuation layer (i.e., there are two structural micro-mechanical layers, the upper mirror layer and the lower actuator layer). Accordingly, the device proposed by Nasiri is relatively complex to fabricate. Moreover, it is expected that the excess mass will lower the resonant frequency.
It is an object of the instant invention to provide a MEMS device wherein the electromechanical design is less integrated with the optical design.
It is another object of the instant invention to provide a MEMS device that is relatively easy to fabricate.
The instant invention provides a MEMS device wherein the actuators are only mechanically coupled to the optical components. For example, in one embodiment, the MEMS device includes an array of micromirrors, wherein the rotation of each micromirror is actuated by one or more electrostatically driven levers that are mechanically coupled to the micromirrors. Preferably, each micro-mirror and/or lever is suspended above the electromechanical substrate via one or more torsional hinges. Advantageously, the micro-mirrors and lever are fabricated from the same layer during the micro-machining process.
In accordance with the instant invention there is provided a micro-electro-mechanical device comprising: a substrate; a mirror supported above the substrate, the mirror including a first plate having a reflective coating deposited thereon and configured to tilt about a first rotation axis; and an actuator for controlling a tilt of the first plate, the actuator including a second other plate supported above the substrate, the second plate configured to tilt about a second other rotation axis, the first and second plates being mechanically coupled via a torsional coupling hinge.
In accordance with the instant invention there is provided a micro-electro-mechanical device comprising: a substrate; a linear array of mirrors suspended above the substrate, each mirror including a mirror plate with a reflective surface deposited thereon, each mirror plate movable about a fixed rotation axis; and an actuator for controlling the movement of each mirror, each actuator including a actuator plate suspended above the substrate, the actuator plate configured to move about another fixed rotation axis, wherein each mirror plate is mechanically coupled to an actuator plate via a torsional spring.
In accordance with the instant invention there is provided a micro-electro-mechanical device fabricated from a micro-machining process, the device comprising: a micro-electronic substrate; a mirror including a first plate having a reflective surface deposited thereon flexibly suspended over the micro-electronic substrate; and an actuator including a second plate flexibly suspended over the micro-electronic substrate and coupled to the first plate via a flexible joint, wherein the first plate, the flexible joint, and the second plate are fabricated from a same layer during the micro-machining process.